Coarse bed material patch evolution in low-order, ephemeral channels

In river channel beds composed of a wide range of grain sizes, the bed material is often arranged in discrete patches discernable by relative texture. These bed material patches are the primary source of entrainable coarse sediment within the channel system and their composition and size have been found to influence the composition and rate of sediment transport. Twelve coarse (gravel–cobble) sediment patches distributed throughout the channel network within a 4.53 ha watershed in southeastern Arizona were monitored for 2 years. Changes in patch area and grain size were measured and painted patch grains were monitored to confirm that patch grains were mobilized during flow. Individual coarse bed material patches exhibited variable persistence during flows with return frequencies ranging from approximately 1 year to 4.6 years. While no patch fully dispersed during the study period, two new patches formed. Most coarse patches remained relatively stable in area and grain-size distribution despite the entrainment of patch grains as lost grains were sufficiently replaced with sediment from upstream. Because of the grain replacement process and the effect of other sediment supply dynamics, the changes in patch area and grain-size distribution display a complex relationship with the magnitude of predicted grain mobilization within each patch. Results indicate that relative stability varies from patch to patch, influenced by the balance of patch grains transported out of the patch and the deposition of new grains into the patch. Predictive models of coarse sediment transport and yield that assume the channel bed is a fixed source of sediment supply may not adequately capture the sediment dynamics within patchy channel beds and should be used with caution when applied to these environments due to the possibility of patch instability as documented in this study.